1. Field of the Invention
The present invention relates to an optical imaging system. More particularly, the present invention relates to an optical imaging system in which extra axial aberrations are minimized.
2. Description of the Prior Art
It is difficult to completely eliminate aberrations in all kinds of optical systems. If a particular aberration is to be removed, another kind of aberration is increased, and therefore, it is difficult to completely eliminate aberrations. In the conventional techniques, most methods which are attempted to eliminate aberrations make efforts to vary the curvature of lenses or to combine various lenses, thereby trying to minimize them. As result of such efforts, the non-spherical lenses and the zoom lenses were developed.
As a method which is different from the above described ones, U.S. Pat. No. 5,013,133 discloses a technique in which a diffractive optical imaging lens system is used for reducing the aberrations in a optical imaging lens system. FIG. 1 is a schematic view showing the conventional diffractive optical imaging lens system which is disclosed in the above patent. This conventional diffractive optical imaging lens system includes a zone plate type of a diffractive lens 10 and by positioning an aperture stop 12, the aberration is reduced in the image which is formed by the diffractive optical lens.
The operation and effect of the method of the above patent will be described below in brief terms.
Diffractive lens 10 consists of a central circular zone having a radius r1, and annular zones formed between radii r1, r2 and r3. Thus diffractive lens 10 provides diffractive zones which are disposed in a parallel relationship, and therefore, diffractive lens 10 has a anamorphic configuration. The optical axis of the diffractive lens passes in the direction of the longest conjugate distance through aperture stop 12 which is disposed separately from the diffractive lens. The direction should desirably lie toward the front focal plane of diffractive lens 10 which is remotely separated from an object. In the case of a paraxial ray in which the object is separated remotely (in other words, at an infinite distance), coma, astigmatism, distortions and field curvatures are reduced to zero. That is, as is described in the above patent, the field aberration of coma, astigmatism and field curvature becomes zero for an object separated in an infinite distance. Further, aperture stop 12 is provided with a phase corrector plate 14 (for example, aspheric corrector plate) so as to correct the spherical aberration.
The optical rays which are disposed outwardly from the paraxial rays generate a phase difference with the paraxial rays when passing through diffractive lens 10. Therefore, in order to correct this, phase corrector plate 14 is installed on aperture stop 12. Phase corrector plate 14 is an aspheric corrector plate of Schmidt camera type. Phase corrector plate 14 is properly designed for correcting the spherical aberration by applying the usual design technique. In the case where the spherical aberration is corrected by using phase correcting plate 14, the incoming optical rays spaced from the paraxial rays generate a phase difference relative to the paraxial rays, but the phase difference disappears when passing through diffractive lens 10.
On the other hand, in the case of finite conjugate rays, sufficient zone intervals and an adjustment of the distance of aperture stop 12 are provided, so that a comaless petzval curvature and a finite conjugate can be obtained.
When eliminating the aberration based on the conventional method, a fixed parameter has to be satisfied and therefore, there is a limit in designing the optical system. Further, curved surfaces having different curvatures exist on one lens, and therefore, the manufacturing of the lens becomes difficult. The most important of all, it is applicable only to a monochrome system, and therefore, it can be applied to only the laser scanner and the like. Therefore, there remains the need for an optical system which is widely applicable and applicable to the general optical fields.